362 FISHES. 



the Common Sturgeon of tlie United States {A. maculosu^, 

 whicli sometimes crosses the Atlantic to the coasts of Great 

 Britain ; Giildenstsedt's Sturgeon {A. gulderistcedtii), common 

 in European and Asiatic rivers, which yields more than one- 

 fourth of the caviare and isinglass exported from Eussia; 

 the Common Sturgeon of Western Europe {A. sturio), which 

 attains to a length of 18 feet, and has established itself also 

 on the coasts of Eastern North America. 



ScAPHiEHYNCBus. — Snout spatulate ; posterior part of the tail 

 attenuated and depressed, so that it is entirely enveloped by the 

 osseous scutes. Spiracles none. The caudal rays do not extend 

 to the extremity of the tail, which terminates in a filament. 



Four species are known : one {S. platyrhynchus) from the 

 river-system of the Mississippi, and the three others from 

 Central Asia; all are exclusively freshwater fishes; their 

 occurrence in so widely distant rivers is one of the most 

 striking instances by which the close affinity of the North 

 American and North Asiatic faunas is proved. 



Second Family — Polyodontid^. 



Body naked, or ivith minute stellate ossifications. Mouth 

 lateral, very wide, with minute teeth in both jaws. Barhels 

 none. Caudal fin with fulcra. Dorsal and anal fins approxi- 

 mate to the caudal. Four gills and a half ; no opercular gill 

 or psetodobraTichia. 



PoLYODON (Spatulabia). — The snout is produced into an ex- 

 ceedingly long, shovel-Iike process, thin and flexible on the sides. 

 Spiracles present. Gill-cover terminating in a very long tapering 

 flap. One broad branchiostegal. Each branchial arch with a 

 double series of very long, fine, and numerous gill-rakers, the two 

 series being divided by a broad membrane. Air-bladder cellular. 

 Upper caudal fulcra narrow, numerous. 



The single species, P. folium, occurs in the Mississippi, 



